Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Bombardier Model DHC-8-100, -200, and -300 series airplanes. This proposal would require installation of a backup pressure regulating valve on the oil pump of the propeller control unit (PCU) on both engines. This action is necessary to prevent a build-up of oil pressure in the oil pump of the PCU should the existing valve fail. Such failure of the pressure regulating valve could lead to oil leaks, fracture of the pump, inability to maintain engine oil pressure, and inability to feather the propeller, with consequent reduced controllability of the aircraft. This action is intended to address the identified unsafe condition.